Presently atomic frequency standard based timekeeping devices like atomic clocks are essential component for precise and accurate scaling and measurement of time in different applications like global positioning system, high speed networking, telecommunication industry or in advance physics experiment. The magnetometer has also become increasingly important in different space, bio-medical and geophysical mapping applications for the measurement of magnetic field. Thus, it is apparent that, atomic timekeeping device and magnetometer both are required in any unique application where atomic frequency standard and high sensitive measurement of magnetic field are simultaneously required.
One of the leading techniques for precise time keeping relies on ultra narrow resonance spectral profile generated by CPT in atomic ensemble by using quantum interference phenomena. Such technique offers unique advantage of miniaturizing the whole device to chip scale as has been demonstrated by S. Knappe, V. Shah, P. D. D. Schwindt, L. Hollberg, J. Kitching, L Liew and J. Moreland, App. Phys. Lett., 85, 1460 (2004). The quantum interference phenomena also provide the physics basis for development of high sensitive chip scale atomic magnetometers as illustrated in P. D. D. Schwindt, S. Knappe, V. Shah, L. Hollberg, and J. Kitching, Appl. Phys. Lett., 85, 6409 (2004).
The CPT in such atomic system is realized by simultaneously coupling two excitation pathways with a common energy level. Here, the ultra narrow spectral profile is generated without using microwave cavity, thereby offering miniaturization of the atomic sensors. The field insensitive transition is used as the atomic frequency reference. The separation between the field insensitive transition and field sensitive transition, measured by scanning the radiofrequency oscillator, gives the value of magnetic field.
The intriguing aspects of polarization rotation of the CPT state in the low field regime, where Larmor's frequency is smaller than the spectral width illustrated interesting features as has been studied by S. Pradhan et al. in App. Phys. Lett., 100, 173502 (2012). According to the said prior art the CPT signal and the polarization rotation signal are significantly enhanced for a Zeeman degenerate system and it has been pointed out in the art that the zero crossing of the CPT profile is insensitive to the applied magnetic field, where as the zero crossing of the polarization rotation signal varies linearly with the field amplitude. Here also measurement of magnetic field requires the scanning of radiofrequency oscillator across the absorptive and dispersive profile of the CPT states.
There are some patent documents exists dealing with such atomic system or device for realizing atomic frequency standard or measuring magnetic field.
U.S. Pat. No. 6,320,472 B1 discloses an atomic frequency standard based on the ground state hyperfine resonance line observed by means of coherent radiation fields creating CPT in an alkali metal atomic ensemble contained in a cell. The CPT state created by a bi-chromatic field is used to stabilize the radiofrequency oscillator, which constitute the atomic clock. This method can only provide atomic frequency standard.
U.S. Pat. No. 6,265,945 B1 also discloses an atomic frequency standard based upon coherent population trapping but similar to the previous system method for operation of the atomic frequency standard of US '945 is not suitable for simultaneous magnetic field measurement.
US 2010/0188081 reports a method and device for measuring magnetic field which makes use of the Zeeman effect for measuring magnetic fields, by way of CPT resonances. The method describes measurement of magnetic field by monitoring the frequency separation between the CPT resonances, thus this technique requires scanning of the radiofrequency oscillator for measurement of magnetic field. This not only become cumbersome, but the device no longer function as an atomic frequency standard.
EP/I029540/1 involves a proposal for development of atomic frequency standard and atomic magnetometer based of coherent population trapping effect in atomic vapour such as Rubidium and Cesium atomic vapour loaded into hallow core optical fibre. This proposal envisaged use of hallow core optical fibre for confining the atomic vapour instead of conventional glass cell. Like conventional CPT based device it cannot simultaneously provide atomic frequency standard and measure magnetic field. It also requires scanning of the radiofrequency oscillator for measurement of magnetic field.
Thus there has been always a need for single atomic device which can provide atomic frequency standard and/or measure magnetic field simultaneously without involving any scanning operation and thus the device should be adapted to be used in any type applications where atomic frequency standard and high sensitive measurement of magnetic field are simultaneously or independently required.